Boston bombings suspect Tamerlan Tsarnaev was on a classified U.S. list of known and potential terrorists, but he wasn’t watched closely, because there are more than 500,000 people in the database.

The 26-year-old, is believed to have carried out last week’s Boston Marathon bombings with his brother, Dzhkokhar Tsarnaev. He was on a list known as the Terrorist Identities Data Environment, Reuters reports, citing sources close to the investigation into the bombings.

Tamerlan was put in the database in 2011 after the FBI spoke to him at the urging of Russian officials concerned he may have become radicalized during a trip to Russia that year. The FBI has since said the interview and a subsequent investigation did not yield any clues that Tamerlan engaged or would engage in terrorist activity.

“The FBI did not find any terrorism activity, domestic or foreign, and those results were provided to the foreign government in the summer of 2011. The FBI requested but did not receive more specific or additional information from the foreign government,” the bureau said in a statement.

The TIDE database contains more than 500,000 names, and not everyone in it gets routinely monitored because of the sheer size of the list, U.S. officials told Reuters.

More than 540,000 names were on the TIDE list as of 2008, according to Reuters, although roughly 90,000 of those were aliases or variant spellings for the same people. Fewer than 5 percent were U.S. citizens or legal residents.

Because Tamerlan was not viewed as an active threat, he was not placed in other databases, such as the no-fly list or the Selectee List, which would have subject him to extra security screening at airports, Reuters reported.

He was also placed on a list compiled by the U.S. Homeland Security Department’s Customs and Border Protection Bureau. The list helps screen for people crossing U.S. borders and entering America via airplane or boat, but Tamerlan was not flagged on the database because the FBI did not deem him a threat.

Bob Grenier, a former chief of the CIA’s Counterterrorism Center, said just because someone’s name appears on a watch list doesn’t mean they’re automatically scrutinized. He said only when there is “a strong piece of information against somebody” do federal authorities keep tabs on a person.

Homeland Security Secretary Janet Napolitano was grilled by Congress Tuesday over the watch-list system and why Tamerlan had not been flagged.

"Yes, the system pinged when he was leaving the United States [in 2011]. By the time he returned, all investigations -- the matter had been closed, " Napolitano told the Senate Judiciary Committee. She also met with the committee in a closed-door session.

Sens. John McCain, R-Ariz., and Lindsey Graham, R-S.C., are among the senators criticizing government officials for not keeping a close eye on Tamerlan.

In particular, Graham lambasted the FBI for not being quick enough to link the Tsarnaevs to the bombings.

"After the bomb went off, don't you think one of the first things the FBI would do is say, 'Have we interviewed anybody in the Boston area that may fit the profile of doing this?' How could his name not pop up, the older brother? And when you have the photo the whole world is looking at, how could we not match that photo with him already being in the system?" Graham said.